The present invention relates to an improvement in fluid machinery having a blade apparatus, such as a steam turbine, a gas turbine, a compressor and a fan, and to a blade apparatus employed in these fluid machineries. More particularly, the invention relates to a blade apparatus for fluid machinery in which the blades are connected by connecting members to each other and to fluid machinery having such a blade apparatus.
In a conventional and generally employed blade apparatus for fluid machinery, such as a steam turbine, a gas turbine, a compressor and a fan, in order to avoid changing the conformation or shape of the blade apparatus itself, and so as to heighten the rigidity function and the vibration damping function, there has been adopted widely a blade connecting structure in which the blades around an outer peripheral portion of a blade wheel are connected to each other by connecting members.
Many kinds of blade connecting structures for the blades in such fluid machinery have been selected and adopted to comply with the demands and the purpose of use. A case of a blade apparatus having a connecting member adopted for use in an axial flow turbine will be exemplified referring to FIG. 2 to FIG. 7.
Since these connecting structures are well known generally, a detailed explanation thereof will be omitted herein. In general, the connecting structure is classified mainly into two basic structures, namely one structure in which tip portions of blades 1 are connected by a shroud or a cover 3a and the root or base portions are connected by a blade wheel 2 to each other (confer from FIG. 2 to FIG. 4), and another structure in which a longitudinal intermediate portion of each blade 1 is connected by a rod 3b or a tie wire 3 (confer from FIG. 5 to FIG. 7).
Within the above blade connecting structures, a case (corresponding to FIG. 5) in which the tie wire 3 is arranged around the periphery of the blade wheel 2 will be shown schematically from FIG. 8 to FIG. 10.
In these figures, to simplify the explanation, the total number M of the blades 1 around the periphery of the blade wheel 2 is twenty (20), namely M=20. Further, two kinds of the tie wires 3 are provided, in which one kind of tie wire 3A is arranged at a certain radial position of the blade 1 and another kind of tie wire 3B is arranged at another certain radial position of the blade 1, namely a case comprising a two-stage connecting structure is shown.
Further, in this case, it is not necessary for the installing radius of the tie wires 3A and 3B to be different, but there is a case in which the tie wires 3A and 3B are arranged at the same installing radial positions and at the same distance. The above stated blade connecting structure having more than two kinds of connecting members is referred to hereinafter as a multiple stage blade connecting blade structure.
In FIG. 8, in the blade connecting structure, each blade group comprises four blades 1 and these blades 1 are connected in each group by a first stage tie wire 3A and a second stage tie wire 3B. Five (5) blade groups are formed around the periphery of the blade wheel 2.
As stated above, the blade structure each blade group is formed by a finite number of blades and such a blade group is arranged repeatedly in the circumferential direction around the blade wheel 2. The above blade structure is referred to in general as a finite blade group or called simply a blade group.
In the case of this figure, each of the double connecting members is provided in the same number for the blade group, and the blade group and all of the cutoff portions (non-connecting portions of the connecting members which form a space between adjacent blade groups) of the blade group are positioned at the same circumferential position.
On the contrary, FIG. 9 shows a blade apparatus in which all of blades 1 around the periphery are connected by a first stage tie wire 3A and a second stage tie wire 3B without the cut-off portions, and hereinafter this blade apparatus is referred to as a whole periphery one ring structure. This figure shows a double whole periphery one ring structure.
FIG. 10 shows a compound structure combining the blade group structure having the tie wires 3A shown in FIG. 8 with the whole periphery one ring structure having the tie wire 3B shown in FIG. 9.
As the prior art relating to the above stated blade apparatuses, Japanese patent laid-open No. 30902/1992 will be listed as one example.
Since the blades of the thus formed blade apparatus are combined respectively, these blades are constituted to provide mechanical security and are valid. Herein, the blade apparatus will be taken under consideration from an aspect of vibration.
Each of FIG. 11 and FIG. 12a-FIG. 12e shows an example of a natural vibration mode in a blade group structure, and in particular FIG. 12a-FIG. 12e show, respectively, a different vibration mode in the case in which the blade group structure is viewed from the radial direction.
Namely, FIG. 11 is a partial view showing the blades 1 and FIG. 12a is a view showing in plane level a condition of a vibration width L.sub.1. Further, along the entire periphery of one ring structure, because of the coupled vibration of all the blades around the periphery, it is well known to have a series of natural vibration mode groups which are referred to as nodal diameter modes, as shown in FIG. 13. In this figure, a dashed line Q shows a nodal line of the vibration.
As to the natural vibration mode of the compound structure (FIG. 10), formed by a combination of the blade group structure and the whole periphery one ring structure, from the relationship of the function according to the present invention, the explanation thereof is omitted herein, however, it will be explained again in later part of this specification.
Further, for purposes of this description, it will be considered that each natural vibration mode resonates by the action of an excitation force on the blade structure. As the excitation force acting on the blades of the fluid machinery, the most popular influence is caused by the fluid.
FIG. 14 is a schematic view of the component of the excitation force in a case in which a flow extending over the whole periphery of the blade wheel is non-uniform.
In accordance with Fourier analysis of the fluid force repeated for every rotation of the turbine, it is well known that the fluid force is divided into an excitation force having a frequency component which corresponds to an integer number times of the rotation speed (hereinafter referred to as an excitation order and expressed by the symbol j).
When the above stated excitation force acts on the blades, a first resonance condition is that the blade natural frequency coincides with the excitation frequency corresponding to an integer number of the rotation speed. In the blade group shown in FIG. 8, it is well known that, so long as the above first resonance condition is satisfied, the blade group can resonate in almost all excitation orders.
The resonance condition of the blades along the entire periphery of the one ring structure as shown in FIG. 9 is also well known. However, in addition to the above stated first resonance condition, it is necessary to note that the following equation will hold as a second resonance condition.
Namely, EQU j.+-.k=.lambda.M (1)
Herein,
.lambda.: zero (0) or positive integer number; PA1 k: nodal diameter number of natural vibration mode of blades having whole periphery one ring structure; PA1 M: total blade number around the periphery. PA1 1: integer number (0.ltoreq.1.ltoreq.n/2); PA1 .epsilon.: integer number (0.ltoreq.1.ltoreq.m/2); PA1 m: constructive blade number of one period structure.
In this specification, the explanation about Equation (1) is not stated in detail; at any rate, when the above stated condition is satisfied, a resonance occurs.
Ordinary fluid machinery generally is designed so that a resonance having a high vibration stress does not occur, or it is designed for affording strength so as not to cause a problem even if resonance occurs.
However, in fluid machinery for performing a load operation extending over a wide rotation speed range, there are many cases in which there is difficulty in designing the fluid machinery so as to avoid resonance for all natural vibration modes of the blades.
Further, in accordance with the dimension and the shape of the blades or the change in the material of the blades, it is frequently the case that the fluid machinery is designed for affording increased strength.
Further, flutter exists as a self-excited vibration of the blades of the fluid machinery. However, flutter differs from the forced vibration produced by the above stated fluid force.
In the case of flutter, since energy is supplied from fluid accompanying minute vibration of the blades, flutter is a vibration which possibly occurs at the rotation speed corresponding to resonance, except for the resonance rotation speed in which the above stated first condition is satisfied.
As a result, taking account of these situations, it is difficult to obtain fluid machinery having a blade apparatus in which a self-excited vibration rarely occurs and in which a high reliability is attained.